virus

virus

virus, parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100–2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes. In one stage of their life cycle, in which they are free and infectious, virus particles do not carry out the functions of living cells, such as respiration and growth; in the other stage, however, viruses enter living plant, animal, or bacterial cells and make use of the host cell's chemical energy and its protein- and nucleic acid–synthesizing ability to replicate themselves.

The existence of submicroscopic infectious agents was suspected by the end of the 19th cent.; in 1892 the Russian botanist Dimitri Iwanowski showed that the sap from tobacco plants infected with mosaic disease, even after being passed through a porcelain filter known to retain all bacteria, contained an agent that could infect other tobacco plants. In 1900 a similarly filterable agent was reported for foot-and-mouth disease of cattle. In 1935 the American virologist W. M. Stanley crystallized tobacco mosaic virus; for that work Stanley shared the 1946 Nobel Prize in Chemistry. Later studies of virus crystals established that the crystals were composed of individual virus particles, or virions. By the early 21st cent. the understanding of viruses had grown to the point where scientists synthesized (2002) a strain of poliovirus using their knowledge of that virus's genetic code and chemical components required.

Viral Structure

Viral Infection of a Host Cell

A free virus particle may be thought of as a packaging device by which viral genetic material can be introduced into appropriate host cells, which the virus can recognize by means of proteins on its outermost surface. A bacterial virus infects the cell by attaching fibers of its protein tail to a specific receptor site on the bacterial cell wall and then injecting the nucleic acid into the host, leaving the empty capsid outside. In viruses with a membrane envelope the nucleocapsid (capsid plus nucleic acid) enters the cell cytoplasm by a process in which the viral envelope merges with a host cell membrane, often the membrane delimiting an endocytic structure (see endocytosis) in which the virus has been engulfed.

Within the cell the virus nucleic acid uses the host machinery to make copies of the viral nucleic acid as well as enzymes needed by the virus and coats and enveloping proteins, the coat proteins of the virus. The details of the process by which the information in viral nucleic acid is expressed and the sites in the cell where the virus locates vary according to the type of nucleic acid the virus contains and other viral features. As viral components are formed within a host cell, virions are created by a self-assembly process; that is, capsomere subunits spontaneously assemble into a protein coat around the nucleic core. Release of virus particles from the host may occur by lysis of the host cell, as in bacteria, or by budding from the host cell's surface that provides the envelope of membrane-enveloped forms.

Some viruses do not kill host cells but rather persist within them in one form or another. For example, certain of the viruses that can transform cells into a cancerous state (see cancer) are retroviruses; their genetic material is RNA but they carry an enzyme that can copy the RNA's information into DNA molecules, which then can integrate into the genetic apparatus of the host cell and reside there, generating corresponding products via host cell machinery (see also retrovirus). Similarly, in bacterial DNA viruses known as temperate phages, the viral nucleic acid becomes integrated into the host cell chromosomal material, a condition known as lysogeny; lysogenic phages are similar in many ways to genetic particles in bacterial cells called episomes (see recombination).

Viral Diseases

Some human diseases are apparently caused by the body's response to virus infection: immune reaction to altered virus-infected cells, release by infected cells of inflammatory substances, or circulation in the body of virus-antibody complexes are all virus-caused immunological disorders. Viruses cause many diseases of economically important animals and plants, some transmitted by carriers such as insects. A retrovirus (HIV) causes AIDS, several viruses (e.g. Epstein-Barr virus, human papillomavirus) cause particular forms of cancer in humans, and many have been shown to cause tumors in animals. Other viruses that infect humans cause measles, mumps, smallpox, yellow fever, rabies, poliomyelitis, influenza, and the common cold.

The techniques of molecular biology and genetic engineering have made possible the development of antiviral drugs effective against a variety of viral infections. Viruses, like bacterial infective agents, act as antigens in the body and elicit the formation of antibodies in an infected individual (see immunity). Indeed, vaccines against viral diseases such as smallpox were developed before the causative agents were known. Some viruses stimulate cellular production of interferon, which inhibits viral growth within the infected cell.

Classification

Bibliography

See C. Zimmer, A Planet of Viruses (2011).

Virus

Any of a heterogeneous class of agents that share three characteristics: (1) They consist of a nucleic acid genome surrounded by a protective protein shell, which may itself be enclosed within an envelope that includes a membrane; (2) they multiply only inside living cells, and are absolutely dependent on the host cells' synthetic and energy-yielding apparatus; (3) the initial step in multiplication is the physical separation of the viral genome from its protective shell, a process known as uncoating, which differentiates viruses from all other obligatorily intracellular parasites. In essence, viruses are nucleic acid molecules, that is, genomes that can enter cells, replicate in them, and encode proteins capable of forming protective shells around them. Terms such as “organism” and “living” are not applicable to viruses. It is preferable to refer to them as functionally active or inactive rather than living or dead.

Electron micrographs of highly purified preparations of some viruses

The primary significance of viruses lies in two areas. First, viruses destroy or modify the cells in which they multiply; they are potential pathogens capable of causing disease. Many of the most important diseases that afflict humankind, including rabies, smallpox, poliomyelitis, hepatitis, influenza, the common cold, measles, mumps, chickenpox, herpes, rubella, hemorrhagic fevers, and the acquired immunodeficiency syndrome (AIDS) are caused by viruses. Viruses also cause diseases in livestock and plants that are of great economic importance. See Acquired immune deficiency syndrome (AIDS), Plant pathology

Second, viruses provide the simplest model systems for many basic problems in biology. Their genomes are often no more than one-millionth the size of, for example, the human genome; yet the principles that govern the behavior of viral genes are the same as those that control the behavior of human genes. Viruses thus afford unrivaled opportunities for studying mechanisms that control the replication and expression of genetic material. See Human Genome Project

Although viruses differ widely in shape and size (see illustration), they are constructed according to certain common principles. Basically, viruses consist of nucleic acid and protein. The nucleic acid is the genome which contains the information necessary for virus multiplication and survival, the protein is arranged around the genome in the form of a layer or shell that is termed the capsid, and the structure consisting of shell plus nucleic acid is the nucleocapsid. Some viruses are naked nucleocapsids. In others, the nucleocapsid is surrounded by a lipid bilayer to the outside of which “spikes” composed of glycoproteins are attached; this is termed the envelope. The complete virus particle is known as the virion, a term that denotes both intactness of structure and the property of infectiousness.

Viral genomes are astonishingly diverse. Some are DNA, others RNA; some are double-stranded, others single-stranded; some are linear, others circular; some have plus polarity, other minus (or negative) polarity; some consist of one molecule, others of several (up to 12). They range from 3000 to 280,000 base pairs if double-stranded, and from 5000 to 27,000 nucleotides if single-stranded. See Virus classification

Viral genomes encode three types of genetic information. First, they encode the structural proteins of virus particles. Second, most viruses encode enzymes capable of transcribing their genomes into messenger RNA molecules that are then translated by host-cell ribosomes, as well as nucleic acid polymerases capable of replicating their genomes; many viruses also encode nonstructural proteins with catalytic and other functions necessary for virus particle maturation and morphogenesis. Third, many viruses encode proteins that interact with components of host-cell defense mechanisms against invading infectious agents. The more successful these proteins are in neutralizing these defenses, the more virulent viruses are.

The two most commonly observed virus-cell interactions are the lytic interaction, which results in virus multiplication and lysis of the host cell; and the transforming interaction, which results in the integration of the viral genome into the host genome and the permanent transformation or alteration of the host cell with respect to morphology, growth habit, and the manner in which it interacts with other cells. Transformed animal and plant cells are also capable of multiplying; they often grow into tumors, and the viruses that cause such transformation are known as tumor viruses. See Retrovirus, Tumor viruses

There is little that can be done to interfere with the growth of viruses, since they multiply within cells, using the cells' synthetic capabilities. The process, interruption of which has met with the most success in preventing virus multiplication, is the replication of viral genomes, which is almost always carried out by virus-encoded enzymes that do not exist in uninfected cells and are therefore excellent targets for antiviral chemotherapy. Another viral function that has been targeted is the cleavage of polyproteins, precursors of structural proteins, to their functional components by virus-encoded proteases; this strategy is being used with some success in AIDS patients. See Cytomegalovirus infection, Herpes, Influenza

Antiviral agents on which much interest is focused are the interferons. Interferons are cytokines or lymphokines that regulate cellular genes concerned with cell division and the functioning of the immune system. Their formation is strongly induced by virus infection; they provide the first line of defense against viral infections until antibodies begin to form. Interferons interfere with the multiplication of viruses by preventing the translation of early viral messenger RNAs. As a result, viral capsid proteins cannot be formed and no viral progeny results.

By far the most effective means of preventing viral diseases is by means of vaccines. There are two types of antiviral vaccines, inactivated virus vaccines and attenuated active virus vaccines. Most of the antiviral vaccines currently in use are of the latter kind. The principle of antiviral vaccines is that inactivated virulent or active attenuated virus particles cause the formation of antibodies that neutralize a virulent virus when it invades the body. See Animal virus, Plant viruses and viroids, Vaccination, Virus, defective

virus

virus

virus

virus

Windows vs. Mac

A Disease - Really?
The concept of a computer "disease" seemed rather foreign in 1989 when this caption from the definition for virus in "The Computer Glossary" was published. Back then, nobody would have believed that millions of viruses were to follow.